ABSTRACT Support is requested for a Keystone Symposia conference entitled B Cell-T Cell Interactions, organized by Drs. Gabriel D. Victora and Carola G. Vinuesa. The conference will be held in Keystone, Colorado from February 10-14, 2019. B cell-T cell interactions bring about the maturation and production of high affinity antibodies and are key to the establishment of effective humoral immunity. While this greatly enhances our ability to fight pathogens, it also can lead to the development of allergies and autoimmune disease. While recent years have seen a burst in our understanding of how B cell-T cell interactions drive antibody affinity maturation, a number of gaps remain, including how these interactions influence the differentiation of B cells into effector fates and how they limit the emergence of autoreactive B cell clones. Most importantly, from the vaccine standpoint, it is likely that better understanding of how T cell help shapes the B cell repertoire will allow us to better control the specificity of vaccine-induced immune responses. The aim of this Keystone symposium will be to: (1) Present the most recent advances in the basic biology of the T cell-dependent B cell response, focusing on Tfh development and function and B cell activation and differentiation; (2) address the gap in our understanding the differentiation of B cells into post-activation (memory and plasma cell) fates; (3) discuss how furthering our understanding of B cell-T cell interactions can inform vaccine development; and (4) present recent findings regarding the role of T- cell dependent B cell response in allergy and autoimmunity.